1. Field of the Invention
The invention is related to the field of combustible gas detectors and, in particular, to a very simple combustible gas detection circuit which produces a uniquely identifiable cricket-like sound upon the detection of a combustible gas.
2. Description of the Prior Art
Combustible gas sensors are commercially available and a wide variety of circuits using these sensors are known in the art. Probably the best known and most widely used combustible gas sensors are the semi-conductor gas sensors manufactured by Figaro Engineering Inc. of Osaka, Japan. A detailed description of these solid state combustible gas sensors and a variety of circuits using the sensors are contained in a paper entitled "Semi-Conductor Gas Sensor" and a brochure entitled "Figaro Gas Sensor TGS 813" available from Figaro Engineering's United States affiliate Figaro U.S.A. Inc., of Wilmette, Ill.
In addition to the circuits shown in the Figaro paper and brochure cited above, Kasahara, in U.S. Pat. No. 3,686,655, discloses two different alarm circuits. In the first circuit, the gas sensor is connected in series with a neon lamp which acts as an oscillator energizing a speaker which is connected in parallel with the neon lamp. In the second circuit, the speaker is connected in series with a Darlington-type amplifier. The gas sensor is part of a voltage divider, the center tap of which is connected to a neon lamp which acts as an oscillator applying an AC current to the base of the Darlington amlifier. Benedict, in U.S. Pat. No. 3,733,595, discloses several circuits in which a gas sensor is part of a voltage divider supplying an actuating potential to the base of a power transistor. The power transistor actuates a relay switch supplying power to a buzzer or a bell. Caillouet, Jr., U.S. Pat. No. 3,750,123, discloses an alarm circuit in which the gas sensor is part of a voltage divider network which supplies the base potential to a power transistor which actuates a relay supplying electrical power to a buzzer. In a circuit similar to that taught by Caillouet, Jr., Paige et al, in U.S. Pat. No. 4,007,456, discloses an alarm circuit having a comparator between the voltage divider and the base of the power transistor.
Ichinose et al, in U.S. Pat. No. 4,170,770, discloses an alarm circuit in which the gas sensor actuates a Darlington-type amplifier in series with a photocoupler. The photocoupler is part of a resistance divider circuit which supplies the trigger voltage to a triac connected in series with a buzzer. The buzzer is connected between the AC input terminals of the circuit. Hall, Jr., is U.S. Pat. No. 4,443,793, discloses a gas detection circuit in which the gas sensor is one branch of a Wheatstone bridge having its opposite output corners connected to the input of a comparator. The output of the comparator is connected to a power transistor connected in series with an alarm. A diode rectifies the output of the transformer to supply a DC current to the Wheatstone bridge, the comparator, and the power transistor. Finally, Buonavita, U.S. Pat. No. 4,480,252, discloses a gas detection circuit in which the Wheatstone bridge receives a regulated DC voltage at its input terminals. The Wheatstone bridge has a pair of gas sensors and the output of the Wheatstone bridge is connected to a comparator which actuates an alarm in response to the detection of the combustible gas.
The invention is a combustible gas detector having a very simple circuit with a minimum number of components and which produces a uniquely identifiable cricket-like sound.